Baluns (unbalanced-balanced converters) are widely known as circuit components for converting unbalanced input/output to balanced input/output or converting balanced input/output to unbalanced input/output.
In recent years, high integration of semiconductor parts such as integrated circuits (IC) has developed, and also miniaturization of the semiconductor parts per se has rapidly advanced. Accordingly, miniaturization of the baluns has been advancing.
As shown as an example in FIG. 12, a conventional balun 100 contains a ½-wavelength unbalanced transmission line 102 and a pair of ¼-wavelength balanced transmission lines 104, 106 (see Patent Document 1).
One end of the unbalanced transmission line 102 has an unbalanced input terminal 108 of the balun 100, and the other end is an open end. One end of each balanced transmission line 104, 106 has a balanced output terminal 110, 112 of the balun 100, and the other end is grounded.
Furthermore, as described in Patent Document 2 and Non-Patent Document 1, improved marchand baluns have been proposed.
In the case of using the improved marchand balun, the even-mode characteristic impedance can be increased, and phase velocities of both modes can be made close to each other, thereby resulting in excellent circuit characteristics.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2002-299127    Patent Document 2: Japanese Laid-Open Patent Publication No. 10-335911    Non-Patent Document 1: IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 41, NO. 12, DECEMBER 1993, pp. 2330-2335 [A Broadband, Planar, Doubly Balanced Monolithic Ka-Band Diode Mixer]